.\" Copyright (C) 2003 Andi Kleen
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.TH ARCH_PRCTL 2 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
arch_prctl \- set architecture-specific thread state
.SH SYNOPSIS
.nf
.B #include <asm/prctl.h>
.br
.B #include <sys/prctl.h>
.sp
.BI "int arch_prctl(int " code ", unsigned long " addr );
.BI "int arch_prctl(int " code ", unsigned long *" addr );
.fi
.SH DESCRIPTION
The
.BR arch_prctl ()
function sets architecture-specific process or thread state.
.I code
selects a subfunction
and passes argument
.I addr
to it;
.I addr
is interpreted as either an
.I "unsigned long"
for the "set" operations, or as an
.IR "unsigned long\ *" ,
for the "get" operations.
.LP
Subfunctions for x86-64 are:
.TP
.B ARCH_SET_FS
Set the 64-bit base for the
.I FS
register to
.IR addr .
.TP
.B ARCH_GET_FS
Return the 64-bit base value for the
.I FS
register of the current thread in the
.I unsigned long
pointed to by
.IR addr .
.TP
.B ARCH_SET_GS
Set the 64-bit base for the
.I GS
register to
.IR addr .
.TP
.B ARCH_GET_GS
Return the 64-bit base value for the
.I GS
register of the current thread in the
.I unsigned long
pointed to by
.IR addr .
.SH RETURN VALUE
On success,
.BR arch_prctl ()
returns 0; on error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
.I addr
points to an unmapped address or is outside the process address space.
.TP
.B EINVAL
.I code
is not a valid subcommand.
.TP
.B EPERM
.I addr
is outside the process address space.
.\" .SH AUTHOR
.\" Man page written by Andi Kleen.
.SH CONFORMING TO
.BR arch_prctl ()
is a Linux/x86-64 extension and should not be used in programs intended
to be portable.
.SH NOTES
.BR arch_prctl ()
is supported only on Linux/x86-64 for 64-bit programs currently.

The 64-bit base changes when a new 32-bit segment selector is loaded.

.B ARCH_SET_GS
is disabled in some kernels.

Context switches for 64-bit segment bases are rather expensive.
As an optimization, if a 32-bit TLS base address is used,
.BR arch_prctl ()
may use a real TLS entry as if
.BR set_thread_area (2)
had been called, instead of manipulating the segment base register directly.
Memory in the first 2GB of address space can be allocated by using
.BR mmap (2)
with the
.B MAP_32BIT
flag.

Because of the aforementioned optimization, using
.BR arch_prctl ()
and
.BR set_thread_area (2)
in the same thread is dangerous, as they may overwrite each other's
TLS entries.

As of version 2.7, glibc provides no prototype for
.BR arch_prctl ().
You have to declare it yourself for now.
This may be fixed in future glibc versions.

.I FS
may be already used by the threading library.
Programs that use
.B ARCH_SET_FS
directly are very likely to crash.
.SH SEE ALSO
.BR mmap (2),
.BR modify_ldt (2),
.BR prctl (2),
.BR set_thread_area (2)

AMD X86-64 Programmer's manual
